1. Field of the Invention
The present invention relates to an image processing method, an image processing apparatus, and an image recording apparatus, and more particularly, to technology for suppressing the thickening of text characters and lines when high density recording is carried out with an image recording apparatus such as an inkjet printer which forms an image composed of dots on a recording medium.
2. Description of the Related Art
An image recording apparatus is known which includes an inkjet head having an arrangement of a plurality of nozzles that eject ink in the form of droplets, and which eject ink droplets from the nozzles toward the recording medium while the inkjet head and the recording medium are caused to move relatively to each other so as to deposit the ejected ink droplets at appropriate positions on the recording medium, and thereby records an image on a recording medium by ink dots formed of the deposited ink droplets.
In an image recording apparatus that records images (including text characters and lines) composed of ink dots in this way, there is a possibility that ink bleeding occurs in the outline areas of the text character or line, thus giving rise to thickening of the text character and line. Consequently, the image quality may be degraded.
Considering this, various techniques have been proposed to prevent the thickening of the text character and line by suppressing ink bleeding in outline areas of the text character and line.
For example, Japanese Patent Application Publication No. 2002-292848 discloses a technique in which the number of dots or the size of dots is reduced, concerning the dots forming an outline that is parallel to the main scanning direction or the dots located inward by one row from the outline. Thereby, the volume of ink is reduced and bleeding of ink in the outline areas is suppressed.
Furthermore, for example, Japanese Patent Application Publication No. 2000-141709 discloses a technique in which it is determined whether the recording pixels in the peripheral pixels around a particular pixel are densely concentrated or not. If the recording pixels are concentrated, then recording is performed using a recording dot with a large diameter. In contrast, if the recording pixels are not concentrated (in the case of text characters or lines where the concentration is low), then recording is performed using a recording dot with a small diameter. Consequently, disruption, and the like, of the text characters are prevented, and the image is made to be sharp.
As described above, in the related art, droplets are ejected to form dots with a large diameter in order to guarantee density and suppress non-uniformities in density, in a solid section. On the other hand, in lines and text characters, or the outline areas thereof, droplets are ejected to form dots with a small diameter in order to suppress thickening of the text characters and lines.
Recently, high density recording has come to be carried out, images being written at a resolution (recording density) of 1200 dots per inch (dpi) or above and small text characters of approximately 4-point size being printed, for instance. Hence, according to these developments, there has been a demand to eject ink droplets having a small dot diameter. If the resolution is 1200 dpi, then a minimum dot diameter must be approximately 21 μm and it is necessary to eject droplets having an ink volume of less than 1 picoliter (pl), in order to make the dot diameter corresponding to the resolution. At present, however, it is difficult to achieve such a dot diameter, and the minimum dot diameter can be around 25 μm to 30 μm.
FIG. 21A shows a case where droplets are deposited to form dots of 25 μm in diameter at a high density of 2400 dpi, for example. FIG. 21A shows an ideal region where recording is expected. In FIG. 21A, the points of intersection of the alternate long and short dash lines, which extend vertically and horizontally, represent the centers of the recording dots. Since the density is 2400 dpi, the interval between the alternate long and short dash lines, which indicates the distance between the centers of the dots, is approximately 11 μm.
The square region enclosed by the solid lines in FIG. 21A indicates the ideal region where recording with the recording dots is expected, in an ideal case where the region of the square shape defined by the dotted lines is recorded with the recording dot positioned at the center of the square shape (the point of intersection of the alternate long and short dash lines). In this case, the ideal dots are square in shape.
FIG. 21B shows a case where dots having a diameter of 25 μm are recorded in such a manner that the centers of the recording dots are positioned at the points of intersection of the alternate long and short dash lines. The regions shown in FIG. 22B that are actually recorded, form a thicker line than the ideal regions where recording is originally expected as shown in FIG. 22A.
In this way, under the conditions of 2400 dpi resolution (namely, a distance between dot centers being 11 μm) and droplet deposition to form dots of 25 μm diameter, if bitmap data for text characters or lines are created on the basis of the resolution, then a thick line exceeding the width of the originally expected line is obtained. If the text characters and lines are broad, then this phenomenon is not problematic; however, in the case of small text characters of 4-point size, or the like, the text characters can become thicker and image quality can decline.
For example, a case is described below in which very small characters are recorded, so that the desired line regions close to each other, as shown by the two rectangular shapes indicated by the solid lines in FIG. 22A. Similarly to the foregoing example, droplets are deposited under the conditions of a resolution of 2400 dpi and a dot diameter of 25 μm. In this case, desirably, the interval between the two regions indicated by the rectangular shapes demarcated by the solid lines is approximately 22 μm (corresponding to 1200 dpi), as shown in FIG. 22A. However, if droplets are deposited to form dots having a diameter of 25 μm at the dot center positions, then the recorded lines become thicker than the desired line regions, and the blank area between the lines (regions) becomes extremely narrow (e.g., 10 μm or less) as shown in FIG. 22B. This leads to a corruption of the text characters, and thus causes the print quality to decline. As described above, in the above-described technology, bleeding is prevented and thickening of the lines is suppressed, by ejecting droplets to form small dots in the outline areas. However, if the minimum dot diameter is larger than the distance between the recording dots (for instance, droplets are deposited to form dots of 30 μm diameter at a resolution of 2400 dpi), then it is difficult to suppress thickening of the text characters and lines.